This invention relates to crystal resonators manufactured to control their acceleration sensitivity.
Piezoelectric quartz crystal resonators have long been employed to develop highly accurate timing signals for such applications as communications, navigation and radar. In particular, resonant frequencies of thickness shear mode quartz resonators are commonly used as timing standards in crystal-controlled oscillators.
In spite of the relative stability and precision of the frequency output of such quartz resonator controlled oscillators, frequency shifts and thus timing errors can occur when the resonator is subjected to acceleration (or gravity) caused stresses. These stresses are produced in the resonator as a result of interaction between the resonator crystal and its mounting or holding structure. Investigations have generally been unsuccessful in identifying effects, or whether there is any effect, of various parameters (crystal geometry, angle of cut, temperature, etc.) on acceleration sensitivity (see Filler et al "Further Studies on the Acceleration Sensitivity of Quartz Resonators", Proc. 37th Annual Symposium on Frequency Control, 1983, pp. 265-271; and Filler, Raymond L., "The Acceleration Sensitivity of Quartz Crystal Oscillators: A Review", paper delivered at 41st Annual Frequency Control Symposium, May, 1987.
A resonator's sensitivity to acceleration has been defined by a so-called "gamma vector". The vector is composed of three frequency shift components which coincide with the x, y & z mechanical (or geometric) axes of the crystal resonator and which are measured for acceleration applied in directions corresponding to each of the axes. Once the gamma vector is known for a crystal, the frequency shift for any acceleration vector a can be obtained as the dot product of that vector a and the gamma vector.
It is desirable, of course, to reduce the magnitude of the gamma vector as much as possible to reduce acceleration-caused frequency error. But, as indicated earlier, there has been little success in doing this in a practical and consistent manner. Crystals prepared and mounted in seemingly an identical fashion can, nevertheless, have different gamma vectors (in both direction and magnitude) for no apparent reason.
One approach for reducing the acceleration sensitivity involves use of two resonators with nearly identical gamma vectors, positioned relative to one another such that their gamma vectors are opposed. See U.S. Pat. No. 4,410,822. In this configuration, the two resonators can be used in series in an oscillator circuit to provide reduced acceleration sensitivity. Another approach uses the signal from an accelerometer placed to measure the acceleration in the direction of the gamma vector to electronically pull the crystal oscillator frequency in a direction to oppose the gamma vector effect. See U.S. Pat. No. 4,453,141.
The first of these methods is impractical because of the aforementioned fact that two crystals, seemingly prepared and mounted identically, have different gamma vectors. Even if one measures the gamma vector for each of the two crystals, the directions of the two gamma vectors will be so random that the physical mounting of the two crystals to position the gamma vectors in opposed fashion will require expensive positioning structure. In addition, the magnitudes of the two gamma vectors may be quite different, which would make the compensation ineffective. The second of these methods requires both expensive positioning of the accelerometer to accommodate an arbitrary direction for the gamma vector, and circuit adjustments to accommodate an arbitrary magnitude of the gamma vector.
A third approach for reducing acceleration sensitivity involves placing the crystal and oscillator circuit in a spring/mass vibration isolation structure to reduce the acceleration magnitude. This approach is limited in that only a certain degree of vibration isolation can be accomplished. In addition, the apparatus used is quite bulky.